The "migration to Glimmer" has been broken out here from #27155 to make the review process less onerous and reduce change risk:
* DEV: migrates most of the widget code to Glimmer in prep for IRV additions
* NB This already incorporates significant amounts of review and feedback from the prior PR.
* NB because there was significant additional feedback relating to older Poll code that I've improved with feedback, there are some additional changes here that are general improvements to the plugin and not specific to IRV nor Glimmer!
* There should be no trace of IRV code here.
Once this is finalised and merged we can continue to progress with #27155.
In the vast majority of cases, people want poll voters to be public. Previously, the checkbox for this was hidden behind the 'show advanced' settings in the poll builder UI.
This commit makes three changes to improve the experience:
1. Add `public=true|false` to poll markup (previously it would only be added when true
2. Bring the 'public' switch outside the 'show advanced' section for improved visibility
3. Change the default to 'true'
